Using code division multiple access (CDMA=Code Division Multiple Access), a multiplicity of data streams may be transmitted at the same time over one common frequency band. In this context, the symbols of the data streams to be transmitted may be modulated using so-called spreading codes.
The data streams transmitted simultaneously using different codes may interfere with each other: multi-path diffusion may lead to the superimposition of sequentially transmitted data symbols (inter-symbol interference, ISI). CDMA coding and multi-path diffusion may cause multiple-user interference (multiple access interference, MAI).
The interferences may be eliminated, e.g., in the receiver, if the impulse response of the channel there is known, as may be described in K. D. Kammeyer: “Message Transmission,” 2nd edition, Information Technology Series, Teubner, Stuttgart, 1996, and from A. Klein, G. K. Kaleh, and P. W. Baier, “Zero Forcing and Minimum Mean-Square-Error Equalization for Multiuser Detection in Code Division Multiple Access Channels,” IEEE Trans. Vehic. Tech., Vol. 45 (1996), 276-287. The channel impulse response may be estimated from a received reference signal, e.g., in the receiver.
To assure a constantly satisfactory transmission, data may be simultaneously transmitted over a plurality of mobile radio channels. This may be achieved using a plurality of transmitting antennas (transmitting antenna diversity). Transmitting antenna diversity of this kind may improve the quality of mobile-radio-channel transmissions in a remarkable manner.
For CDMA mobile radio systems in the TDD mode, the following transmitting antenna diversity schemes may be conventional for the forward link (downlink), thus, e.g., from a base station to a mobile station.
In the Transmit Adaptive Array (TxAA) according to Motorola: “Transmit Diversity Schemes applied to the TDD mode (II),” 3GPP TSG RAN WG1 document TSGR1#5(99)632, the signals of the individual users, before their summation in the baseband, are modified using a phase and amplitude factor.
In the Selection Transmit Diversity (STD) according to Motorola: “Transmit Diversity Schemes applied to the TDD mode (II),” 3GPP TSG RAN WG1 document TSGR1#5(99)632, the signals of the individual users may be always emitted over only one antenna. For different users, different antennas may be selected.
In the Phase Alignment Transmit Diversity (PATD) according to Motorola: “Transmit Diversity Schemes applied to the TDD mode,” 3GPP TSG RAN WG1 document TSGR1#3(99)186, the overall antenna signals may be modified in the baseband using a phase factor. The factor may be different for different antennas. It may be identical for all users on one antenna.
In all of these schemes, the transmission qualities of the mobile radio channels in the counter link may be determined in the transmitter. On the basis of the measured qualities, the parameters of the schemes may then be selected (Closed Loop Technology).
To be able to detect data transmitted using TxAA or STD in accordance with a Joint Detection Method (JD), it may be required for every user to perform its own channel estimation in the receiver.
When PATD is used, it may only be required in the forward link to estimate one channel in the JD receiver. However, in contrast to TxAA and STD, PATD may not employ any amplitude modifications for optimizing the transmission.
Customary diversity methods for the CDMA transmission in the forward link in the TDD mode therefore either may require great expense for channel estimation in the receiver (TXAA=Transmit Adaptive Array, STD=Selection Transmit Diversity), or they may be further improved qualitatively (PATD=Phase Alignment Transmit Diversity).